Two component thermosetting polyurethanes are widely used in protective coatings or films in a broad range of applications, such as for example, coatings for automotive machinery, equipment, and other surfaces in need of a protective coating. These room temperature-curable polyurethanes are prepared from polyisocyanates and a component containing an active hydrogen compound, such as a polyol or an amine.
Two-component polyurethane coating systems include a polyisocyanate component that reacts with a polyol component that comprises, for example, an acrylic polyol or polyester polyol, to form useful films. The system also includes organic solvents, and a variety of adjuvant components, e.g., surface active agents, pigments, dispersants, diluents, and fillers. This type of coating is one of the finest coatings available that can be produced without the application of heat or other external sources of energy. They are very useful for objects that cannot be heat-cured, such as large machinery, airplanes, ships and vehicles.
In some embodiments, two component polyurethane compositions are cured at elevated temperature. It has been recognized that such systems tend to discolor at elevated temperatures and prior art methods to stabilize the color of such polyurethanes systems have been developed. The approach of these methods is either purification or stabilization of the polyisocyanate. For example, U.S. Pat. No. 4,297,472 teaches adding a small amount of vicinal diols to the formulation to improve the color stability of the polyurethanes. U.S. Pat. No. 4,065,362 teaches a process to purify the organic isocyanate, U.S. Pat. No. 3,715,381 teaches incorporating 2,4-di(t-butyl)-p-cresol (BHT) in the formulation, and U.S. Pat. No. 2,957,903 teaches incorporating an amount of triaryl phosphite in a polyisocyanate. U.S. Pat. No. 4,677,154 relates to elimination of discoloration in polyurethanes, particularly thermoplastic polyurethanes, by the addition to the reaction mixture from 0.01 to 1 percent by weight, based on the isocyanate component, a stabilizer package characterized as BHT and a compound selected from a specific group of compounds which includes tris(nonylphenyl) phosphite, distearyl thiodipropionate, triisodecyl phosphite, trilauryl trithiophosphite, and blends thereof.
Other two component polyurethane systems are curable at relatively mild temperatures in the presence of a catalyst for promoting reaction of the polyisocyanate with the polyol. In one embodiment of such low temperature curing two component polyurethane coating systems, the catalyst is included in the polyisocyanate component of the system to form a “hardener” component that is subsequently combined with the polyol component to form a curable coating composition. However, it has been found that including the catalyst in the polyisocyanate component of such systems markedly increases the potential for undesirable discoloration of the polyisocyanate and related coating and despite the known color stabilizing methods, it has been found that a need exists for a stabilizer that will efficiently stabilize the color of a solution of an isocyanate in the presence of a catalyst for the reaction of the isocyanate with an active hydrogen compound.